1. Field of the Invention
This invention relates generally to implantable medical devices, and, more particularly, to methods, apparatus, and systems for providing a variable output ramping signal associated with delivering a therapeutic electrical signal to a target portion of a patient's body.
2. Description of the Related Art
There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, such as epilepsy and other motor disorders, and abnormal neural discharge disorders. One of the more recently available treatments involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied to target locations in the human body to provide various benefits, including reducing occurrences of seizures and/or improving or ameliorating other conditions. A particular example of such a treatment regimen involves applying an electrical signal to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is hereby incorporated in its entirety by reference in this specification. Electrical stimulation of the vagus nerve (hereinafter referred to as vagus nerve stimulation therapy or VNS) may be provided by implanting an electrical device underneath the skin of a patient and performing a detection and electrical stimulation process. Alternatively, the system may operate without a detection system once the patient has been diagnosed with epilepsy, and may periodically apply a series of electrical pulses to the vagus (or other cranial) nerve intermittently throughout the day, or over another predetermined time interval.
When delivering therapeutic electrical signals to neural structures in a patient's body, there may be a concern that a sudden burst of energy provided by the stimulation signal may not be tolerated well by the patient's body. For example, a sudden initiation of an electrical signal may cause an undesired reaction or side effect (e.g., pain). In addition to undesired side effects, a sudden burst of energy resulting from an electrical signal may not produce the intended therapeutic effect. Without being bound by theory, the targeted neural structure may not react properly (e.g., improper processing of the stimulation signal) when a sudden electrical signal is applied. Further, the signal may cause excessive neural conditioning, wherein the target may become conditioned to accept the electrical signal and as a result may not react as expected. This may reduce the efficacy of the therapy being delivered to the patient's body.
In an attempt to alleviate such problems, designers have provided for a ramping time period immediately before and/or after the delivery of the therapeutic electrical signal, in which the electrical signal applied to the target neural structure is gradually increased prior to delivery of the therapeutic dosage of the electrical signal (“ramp-up”) or decreased after its delivery (“ramp-down”). State-of-the-art neurostimulators utilize the ramp-up or ramp-down periods to provide a gradual transition from the delivery of no electrical signal to the delivery of the full therapeutic dosage of the electrical signal (ramp-up) and/or from delivery of the full therapeutic dosage to the delivery of no signal (ramp-down). The ramp-up and ramp-down periods are thus used as a buffer to prepare the target neural structure to receive a full dosage electrical signal. The ramp-up and ramp-down periods provide a transition for the human body to enter or exit both a state of no stimulation and a stimulation state.
There are various limitations associated with state-of-the-art ramping-up and ramping-down methods. Most significantly, the ramp-up and ramp-down process may increase a likelihood of neural conditioning, in which the target structure becomes conditioned to accommodate the therapeutic electrical signal, with the result that therapeutic efficacy of the electrical signal may be diminished. Therefore, the state-of-the-art ramping-up and the ramping-down processes may promote significant neural conditioning that may lead to a reduction in the efficacy of stimulation treatment.
Generally, state-of-the-art ramp-up and ramp-down signal-portions call for ramping-up or ramping-down of the current amplitude of the electrical signal over a fixed duration (e.g., approximately 2 seconds), with fixed step sizes (e.g., 0.25 milliamps per step). This constant ramp-up and ramp-down signal may cause the target neural structure to become conditioned to receiving such constant, steady signal patterns, with a consequent reduction in therapeutic efficacy.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.